The present invention relates to methods and apparatus for controlling gas flows and more particularly, to methods and apparatus for controlling the flow of inert gas to the entrance and/or exit of a chamber which is to be isolated from ambient atmosphere.
It is frequently necessary to treat materials under inert, non-oxidizing atmospheres. In certain treatment processes, for example the curing of solvent borne resin coatings on materials (as illustrated in U.S. Pat. No. 4,150,494 assigned to the assignee of the present invention), such coatings are cured by evaporating solvent in a curing oven. By maintaining an essentially non-oxidizing or inert condition therein, relatively high solvent vapor partial pressures may be safely obtained which facilitates the recovery of such solvent vapors. In order to enable the continuous passage of materials bearing such coatings through a curing oven, appropriate entrance and exit structures such as vestibules or gas curtains are frequently provided. Although such vestibules must be inerted, there is an unavoidable loss of inert gas from the oven to ambient atmosphere in order to preclude the entry of atmospheric or ambient oxygen therein. Inert gas may be supplied to an entrance and an exit vestibule and be removed from the curing oven together with solvent vapor. A mass balance between the supplied inert gas on the one hand and the inert gas leaving the vestibules and curing oven on the other hand will be established. However, it has been found that due to transient ambient air currents or changes in drag forces caused by passage of different coating bearing items through the oven, ambient air may enter and leave the oven notwithstanding maintenance of an inert gas mass balance. Thus, in order to assure exclusion of ambient air (oxygen) from such ovens, structure for monitoring oven conditions must be provided.
As described in U.S. Pat. No. 4,150,494, it is known to monitor pressures in an oven vestibule and control the flow of inert gas thereto in response to such pressures. Thus, should the pressure in a vestibule decrease, the flow of inert gas thereto is increased thereby essentially precluding a flow or diffusion of ambient oxygen into such vestibule. It has been found, however, that the use of pressure transducers in curing ovens is not always reliable and frequently, excessive flows of inert gas have been utilized to assure that enough inert gas is available in or at an oven entrance and exit to preclude entry of ambient oxygen. Such excessive inert gas flows, however, clearly reduce the economic attractiveness of solvent recovery systems.
In equipment adapted to enable radiation curing of coatings on a material passed through an appropriate curing chamber, it has been proposed (U.S. Pat. No. 4,118,873 which is also assigned to the assignee of the present invention) to discharge inert gas flows into such curing chamber with a momentum selected such that internal drag forces are substantially balanced thereby reducing the flow and diffusion of ambient oxygen into such curing chambers. It has been found that by balancing drag forces, coatings on materials passed through the chamber may be cured even though these materials are passed through such chambers at speeds of 1000 ft/min or greater. At such relatively high speeds, significant drag forces are developed and consequently, a momentum balance is helpful in avoiding the loss of excessive flows of inert gas.
Consequently, a clear need exists for an active and reliable technique for controlling the supply of inert gas to a chamber such that the entry of atmospheric oxygen is substantially precluded without the use of excessive and unnecessarily large inert gas flows.